1. Field of the Invention
The present invention relates to a recording apparatus, and more particularly to a serial type recording apparatus in which recording is made while moving a recording head relative to a recorded material by using a brushless motor, e.g., a stepping motor, as a drive power source.
2. Related Background Art
In brushless motors, it is usual that a Hall device or the like is used to perform position detection of a rotor's magnetic pole for control of current supply, and an optical or magnetic encoder is used to perform speed detection of the rotor.
Such brushless motors have, however, suffered from the following problems.
(1) Alignment is required between a stator's magnetic pole and the Hall device; and
(2) With the Hall device used to carry out switching of the current supply, the relative position relationship between the Hall device and the stator is uniquely determined, thus making a current supply method for the motor fixed. Between the case of so-called 180.degree. current supply control and the case of so-called 90.degree. current supply control, for example, the position of the Hall device is electrically different 45.degree. relative to the stator's magnetic pole. In order to perform two modes of current supply control using a single motor, therefore, the number of Hall devices must be doubled and those Hall devices must be arranged at positions respectively suitable for the different modes of current supply control.
Although Japanese Patent Laid-Open No. 62-193548 and No. 62-193549, for example, propose a stepping motor to perform control of the current supply using an encoder output, those publications disclose only the structure of a motor, per se, with an encoder mounted at a predetermined location, and nothing is referred to a drive control circuit for the motor, a control method, etc.
Therefore, U.S. Pat. No. 4,963,808 proposes a motor controller for a stepping motor in which an encoder having detected portions integer times as many as the number of magnetic poles of the rotor is fixed to a rotor shaft, and the number of the detected portions of the encoder having passed over during rotation of the rotor is counted at a predetermined location on the stator side, thereby switching the mode of current supply to a stator coil when the counted value comes into match with a predetermined value.
Meanwhile, drive control of stepping motors have been conventionally made under simple open-loop control of the number of drive pulses for the motor and the frequency of those drive pulses. In the case of using the stepping motor as a carriage drive motor and driving the motor under open-loop control, however, piercing harsh noises occur due to rotor vibrations of the stepping motor, particularly of the hybrid type, while the carriage is being driven to run. Further, when the carriage is started, stopped and reversed in its driving direction, i.e., when the stepping motor is started, stopped and reversed in its rotating direction, large noises like bangs occur because the stepping motor is started and stopped while undergoing vibrations. Those noises raise a problem specially in quiet printers such as ink jet printers represented by bubble jet printers.
In view of the above, U.S. Pat. No. 4,928,050 proposes a recording apparatus in which a stepping motor is used as a drive power source to move a recording head for recording scan, the apparatus comprising detection means for detecting a rotated angular position of a rotor of the stepping motor, and control means for controlling drive of the stepping motor in a closed-loop manner dependent on the detected result of the detection means.
The closed-loop control of the stepping motor, however, requires an encoder for detecting the rotated angular position of the rotor, and also necessitates alignment to be made during assembly of the motor between magnetic poles of the rotor and magnetic poles of the encoder (e.g., slits of a magnetic or optical encoder). The reason of necessitating such alignment is that phase switching of the motor is synchronized with output pulses of the encoder. Unless the alignment is performed with a satisfactory degree of accuracy, the motor may fail to rotate or may rotate at different speeds dependent on the direction.
On the other hand, if the number of output pulses per rotation of the encoder is increased to raise resolution for each pulse, the above alignment can be dispensed with. In the case of a PM type stepping motor which makes a turn through 48 steps, for example, a rotor has 24 magnetic poles. If the encoder is set to output 288 pulses per rotation, there is produced an output of 12 pulses for each of the rotor's magnetic poles. Since a deviation between the center of the rotor's magnetic pole and the center of the encoder's magnetic pole, as resulted when mounting the encoder to the rotor shaft at random, is a half of the pulse interval at maximum, the deviation is within a range of +4.2%. A corresponding deviation of the timing in switching of the excitation current is reduced down to a negligible value.
In that case, it must be decided at the beginning which magnetic pole of the encoder is made correspondent to the rotor's magnetic pole. To this end, a current is first supplied to the motor coil over a predetermined period of time. Subsequently, when the rotor is slightly rotated upon excitation of the motor coil due to the current supply and then stopped, the encoder's magnetic pole confronting the rotor's magnetic pole is selected as a reference. The other encoder's magnetic poles may be successively selected with an interval of 12 pulses, starting from the reference magnetic pole first determined.
The above-stated initialization of encoder pulses must be carried out prior to movement of the motor. In other words, when such a stepping motor is used as a carriage drive motor of a serial printer, it is required to perform an initial operation when a current is applied to the machine body.
With an aim to implement such an initial operation, U.S. Ser. No. 413,473 filed on Sep. 27, 1989, proposes a recording apparatus in which a stepping motor is used as a drive power source to move a recording head for recording scan, the apparatus comprising detection means for detecting a rotated angular position of a rotor of the stepping motor, and control means for controlling drive of the stepping motor in a closed-loop manner dependent on the detected result of the detection means, and for making an operation to drive the stepping motor and hold the rotor under current control with pulse width modulation during the initial process which includes a step of driving the stepping motor under open-loop control.
Meanwhile, some ink jet recording apparatus, for example, are arranged to carry out a wiping operation through engagement with the ink discharge port defining surface of a recording head upon running of a carriage, to thereby clean the head. For the purpose of such a wiping operation, the carriage is required to run at a predetermined speed. Specifically, it is usually required for the carriage to run at a speed considerably lower than that during the running for recording. Also, the running distance through which the carriage is to be run for that purpose is required to be as short as possible from the standpoints of reducing the apparatus size and improving a total printing speed.
Realizing a predetermined speed certainly at a predetermined position in such a short distance is very difficult under closed-loop control using a stepping motor.
There is further known a recording apparatus which has a drive force transmission switching mechanism capable of switching a single drive power source so as to produce drive forces for plural kinds of operations, with a view of reducing the cost. Japanese Patent Laid-Open Application 3-5181 discloses a mechanism for switching the drive force of a sheet feed motor to be transmitted for sheet feeding, drive of an auto sheet feeder, drive of restoring means in an ink jet recording apparatus, and so forth, the mechanism being arranged to switch transmission paths of the drive force dependent on the stop position of a carriage. The recording apparatus having such a mechanism may suffer from the problem that a control system becomes complicated if it is driven under closed-loop control, because the carriage is often stopped for a long time at a predetermined position or operated to reciprocate over a short distance at a predetermined speed.
In a conventional recording apparatus, a stepping motor is driven by switching excitation at the predetermined timing using a predetermined current value. In need of large drive torque, for example, the drive torque is increased by setting the current value to be large or delaying the timing to switch excitation. As regards to an excitation method, the drive torque can be increased by adopting a 2-phase excitation technique rather than a 1-phase excitation technique. In such 1-phase and 2-phase excitation techniques, the torque produced at the time of switching the phase is constant if the current value for each phase is kept constant.
In need of fine accuracy for the stop position, by way of example, adopting 1-2-phase excitation drive or half-step drive can theoretically improve resolution of the stop position twice as much as that obtained by 1-phase or 2-phase excitation drive. Taking a motor which makes a turn through 48 steps as one example, the 1-phase or 2-phase excitation drive provides resolution of 1 step=7.5.degree. and the 1-2-phase excitation drive provides resolution of 1 step=3.75.degree.. When the 1-2-phase excitation drive is used to provide higher resolution, there arises the problem that since the produced torque is larger during the 2-phase excitation than during the 1-phase excitation, the torque during the 2-phase excitation becomes too large if the torque necessary for stopping is set to be suitable for the 1-phase excitation and, conversely, the torque during the 1-phase excitation becomes too small to keep stability if the torque necessary for stopping is set to be suitable for the 2-phase excitation.
Further, the 1-2-phase excitation drive is sometimes adopted so as to provide smoother rotation than the 2-phase excitation drive and to eliminate noises. This case also is accompanied with the problem that fluctuations occur in rotation due to a torque difference between the 1-phase excitation drive and the 2-phase excitation drive, and these torque fluctuations necessarily generate noises.
Moreover, if it is intended to produce sufficient drive torque, taking into account the fact that the motor tends to be out of synchronism during the 1-phase excitation drive due to lack of the drive torque, the torque during the 2-phase excitation becomes too large, thereby causing the problem of noises, etc.